In the field of endodontics, one of the most important and delicate procedures is that of cleaning or extirpating a diseased root canal to provide a properly dimensioned cavity while essentially maintaining the central axis of the canal for filling of the canal void and capping of the tooth. When done properly, this step enables substantially complete filling of the canal with biologically inert or restorative material without entrapping noxious tissue in the canal that could lead to failure of the therapy.
In a root canal procedure, the dentist removes diseased tissue and debris from the canal prior to filling the canal with a biologically inert or restorative filling material. In performing this procedure, the dentist must gain access to substantially the entire canal, shaping it as appropriate, in order for the procedure to be a long-term success. However, root canals often are very small in diameter, and they are sometimes quite curved. It is therefore often very difficult to gain access to the full length of the canal and to work all surfaces of the canal wall.
Many tools and techniques have been designed in an effort to enable dentists to perform the difficult task of cleaning and shaping root canals. Historically, dentists have used endodontic files to remove the soft and hard tissues in and adjacent the root canal. These endodontic files are typically made by grinding helical flutes into a working portion of a small, elongate tapered rod to create a curvilinear, abrasive file with one or more helical cutting edges. FIG. 1 is a partial cross section of a tooth 10 and supporting root structure illustrating the use of a typical fluted endodontic file 16 to carry out a standard root canal procedure. The root canal 12 of a tooth houses the circulatory and neural systems of the tooth. These enter the tooth at the apical terminus of each of its roots 20 and extend through a narrow, tapered canal system to a pulp chamber adjacent a crown portion 22 of the tooth 10. If the pulp tissue within the canal 12 becomes diseased or injured, it can cause severe pain and trauma to the tooth, sometimes necessitating extraction of the tooth. Root canal therapy involves removing the diseased tissue from the canal 12 and sealing the canal system in its entirety. If successful, root canal therapy can effectively alleviate the pain and trauma associated with the tooth so that it need not be extracted.
To perform a root canal procedure, the endodontist first drills into the tooth 10 to locate the root canal(s) 12 and then uses an endodontic file 16 to remove the decayed, injured or dead tissue from the canal 12. The endodontic files are rotated and/or reciprocated within the root canal either by hand or using a slow speed dental handpiece. The primary goal is to remove all of the decayed or injured nerve while leaving the integrity of the root canal walls relatively unaffected. Preserving the integrity of the root canal 12 is important in order to allow proper filling of the root canal void in a homogenous three dimensional manner such that leakage or communication between the root canal system and the surrounding and supporting tissues of the tooth 10 is prevented. Once as much of the diseased material as practicable is removed from the root canal 12, the canal system is sealed closed, typically by reciprocating and/or rotating a condenser instrument in the canal to urge a sealing material such as gutta-percha into the canal.
During use of an endodontic file 16 to perform root canal procedures, cutting surfaces on the instruments remove, chip, and/or abrade material from the root canal as the instrument is rotated and/or reciprocated roto-axially. The cutting surfaces are arranged along a working portion of the instrument which is typically 16 mm in length, approximately the same length as a standard root canal. This rotational movement of the cutting surfaces within the canal creates torsional and other stresses in the instrument body, which can cause a portion of an instrument to break off in the root canal when the instrument becomes over-torqued and/or fatigued.
Typical endodontic files are provided with taper rates of from 0.02 mm/mm to 0.12 mm/mm, with a taper step of 0.02 mm/mm between instruments. Due to the curved and often asymmetrical shape of many root canals, a plurality of instruments must often be employed in series, often in an order from narrow diameter, less tapered instruments to wider diameter, more tapered instruments. A wider, more tapered instrument typically would not be used first in a root canal or other similar procedure because, among other things, the more tapered instrument would not likely reach deep enough into the affected root canal before the wider end of the instrument becomes jammed in the upper hard edge of the upper portion of the canal being operated on. However, as a dentist moves between instruments having the 0.02 mm/mm taper step, the wider portion near the top of the canal still often binds in the canal. This phenomenon is sometimes referred to as “taper lock” and is preferably avoided because it can lead to tooth cracking and other similar complications.
Additionally, when an instrument is urged into the root canal in an excessively forceful manner, the shank, which may be free of cutting surfaces but is typically tapered at the same taper rate as the working portion of the instrument, may engage the upper portion of the root canal applying excessive lateral forces which can cause the tooth to split. This wedging action can also limit the ability of the working portion of the endodontic instrument to reach sufficiently into the root canal to perform its desired work on the canal wall. Further, the shank of conventional tapered endodontic instruments have a greater cross sectional diameter than the working portion and is therefore less flexible, which can interfere with an endodontist's proper maneuvering of the endodontic instrument, especially when it is necessary to roto-axially reciprocate the instrument while attempting to work deeper portions of a canal wall.
Accordingly, there is a need for an endodontic instrument design and method for use that will minimize the wedging action caused by the shank of typical endodontic instruments, decrease the number of instruments required to successfully carryout the procedure, and increase the flexibility of the shank of the endodontic instrument(s).